The present invention relates generally to four-wheel drive motor vehicles and, more particularly, to a transfer case equipped with a disconnectable clutch assembly.
In view of increased demand for four-wheel drive vehicles, a plethora of power transfer systems are currently being incorporated into vehicular drivetrain applications for transferring drive torque to all four wheels. In many vehicles, a transfer case is interconnected between the primary and secondary drivelines which is equipped with a dog-type mode clutch that can be selectively engaged for rigidly coupling the secondary driveline to the primary driveline to establish a part-time four-wheel drive mode. When the mode clutch is disengaged, all drive torque is delivered to the primary driveline for establishing a two-wheel drive mode.
A modern trend in four-wheel drive vehicles is to equip the transfer case with an electronically-controlled transfer clutch in place of the mode clutch. The transfer clutch is operable for automatically directing drive torque to the secondary wheels, without any input or action on the part of the vehicle operator, when traction is lost at the primary wheels for establishing an on-demand four-wheel drive mode. Typically, the transfer clutch is a multi-plate clutch assembly installed between the primary and secondary output shafts of the transfer case and which is actuated by a power-operated actuator in response to control signals sent from a controller. The control signals are based on current operating characteristics of the vehicle (i.e. vehicle speed, interaxle speed difference, steering angle, etc.) as detected by various sensors. Thus, such "on-demand" transfer cases can utilize adaptive control schemes for automatically controlling torque distribution during all types of driving and road conditions. In many instances, the vehicle operator is also permitted to select between the two-wheel drive mode and the part-time four-wheel drive mode in addition to the on-demand four-wheel drive mode. Specifically, when the two-wheel drive mode is selected, the clutch assembly is released for delivering all drive torque to the primary output shaft. In contrast, when the part-time four-wheel drive mode is selected, the clutch assembly is fully engaged for effectively locking the secondary output shaft to the primary output shaft. Examples of such four-wheel drive systems are disclosed in commonly owned U.S. Pat. Nos. 5,323,871, 5,363,938, 5,702,321 and 5,725,453.
While many on-demand clutch control systems similar to those described above are currently being used in four-wheel drive vehicles, a need exists to advance the technology and address recognized system limitations. For example, when the on-demand transfer cases is operating in its two-wheel drive mode, the viscous drag generated by relative rotation between the interleaved clutch plates of the clutch assembly causes the secondary output shaft to rotate which, in turn, results in efficiency losses and reduced fuel economy. This situation is particularly exacerbated during cold weather conditions when the viscosity of the fluid in the transfer case is relatively high. Thus, it would be desirable to equip an on-demand transfer case with a transfer clutch that eliminates or substantially reduces such efficiency losses due to clutch drag.